A. Technical Field
The present invention relates to a water-absorbent resin composition as favorably used for sanitary materials such as paper diapers (disposable diapers), sanitary napkins, and so-called incontinence pads. More specifically, the invention relates to: a water-absorbent resin composition containing a water-absorbent resin granule which is obtained by mixing a water-absorbent resin powder with an aqueous liquid and has high granulation strength and high absorption capacity under a load; and a process for producing the above granule.
B. Background Art
In recent years, water-absorbent resins are widely utilized as constituents of sanitary materials, such as paper diapers, sanitary napkins, and so-called incontinence pads, for the purpose of allowing the water-absorbent resins to absorb body fluids.
As to the above-mentioned water-absorbent resins, the following materials are, for example, known: crosslinked matters of partially neutralized polyacrylic acids, hydrolysates of starch-acrylic acid graft polymers, saponified products of vinyl acetate-acrylic ester copolymers, hydrolysates of acrylonitrile copolymers or those of acrylamide copolymers, or crosslinked matters of these copolymers, and crosslinked matters of cationic monomers.
It is said that the above-mentioned water-absorbent resins should be excellent in the following properties: the water absorption capacity, the water absorption speed, the liquid permeability, the gel strength of hydrogel, the suction power to suck up water from a base material containing an aqueous liquid, and so on, upon contact with an aqueous liquid such as a body fluid. However, relations between these properties do not necessarily display positive correlations. For example, as the absorption capacity increases, some other properties such as liquid permeability, gel strength, and absorption speed deteriorate.
As to a method for improving the above-mentioned water-absorption properties of a water-absorbent resin in good balance, an art in which the neighborhood of the surface of the water-absorbent resin is crosslinked is known, and various methods have been disclosed so far, and many crosslinking agents and conditions have been proposed.
For example, methods are known in each of which the following are used as crosslinking agents: polyhydric alcohols (JP-A-58-180233 and JP-A-61-016903); polyglycidyl compounds, polyaziridine compounds, polyamine compounds, or polyisocyanate compounds (JP-A-59-189103); glyoxal (JP-A-52-117393); polyvalent metals (JP-A-51-136588, JP-A-61-257235 and JP-A-62-007745); silane coupling agents (JP-A-61-211305, JP-A-61-252212, and JP-A-61-264006); alkylene carbonates (DE 4020780).
In addition, as to the crosslinking conditions, other methods are also known in which the following are allowed to be present during a crosslinking reaction: inert inorganic powders (JP-A-60-163956 and JP-A-60-255814); specific dihydric alcohols (JP-A-01-292004); water along with ether compounds (JP-A-02-153903); alkylene oxide adducts of monohydric alcohols, or organic acid salts, or lactams (EP 555692).
On the other hand, generally, as to a water-absorbent resin, it is preferable that the content therein of a powder with a particle diameter of not larger than 150 .mu.m (i.e. fine powder) is as low as possible. The fine powder clogs even in absorbent articles such as diapers and therefore lowers the liquid permeability. In addition, there are problems in that the fine powder is lost as dust when handled, and further in that the properties such as absorption capacity under a load are difficult to improve even if the fine powder is treated by the above-mentioned surface-crosslinking. Thus, a water-absorbent resin containing only a small amount of fine powder is desirable.
Conventional known methods for producing the water-absorbent resin containing only a small amount of fine powder are, for example, as follows: (1) a method comprising adjustment of a particle size by an optimization of the degree of polymerization or pulverization; and (2) a method comprising classification and removal of the formed fine powder with a sieve or a gas current (U.S. Pat. No. 4,973,632).
However, method (1) above gives a large amount of fine powder (ten and several percent to tens of percent) in production process steps. In addition, the abolition of the fine powder as produced in method (2) above results in the much lowering of yields and the disadvantage in the abolition cost.
Thus, various proposals have been made to resolve the above-mentioned problems by granulating or regenerating the fine powder as inevitably formed in production processes for water-absorbent resins.
For example, EP 0463388A, U.S. Pat. Nos. 4,950,692 and 4,970,267, EP 0417761A, and EP 0496594A propose methods (as means other than granulation) for regenerating the fine powder as large particles by pulverizing and then drying a gel which is formed by mixing the fine powder with water or a hydrogel. In addition, EP 0644224 proposes a granulation method comprising the step of carrying out granulation by adding an aqueous solution of a water-soluble or water-dispersible polymer to a water-absorbent resin in the presence of an insoluble inorganic fine powder such that the water content of the resultant granule can fall in the range of 30 to 70% by weight. U.S. Pat. No. 5,002,986, EP 0318989B, U.S. Pat. No. 5,248,709, U.S. Pat. No. 4,123,397, U.S. Pat. No. 4,734,478, and U.S. Pat. No. 5,369,148 propose methods for increasing the average particle diameter of the fine powder to some hundreds of micrometers by granulating the fine powder alone of about 150 micrometers to some tens of micrometers or a powdery mixture thereof with larger particles by using a binder such as an aqueous liquid in an amount of several percent to twenty and several percent of the powder.
However, it has been difficult to uniformly add an aqueous liquid to a water-absorbent resin fine powder because its absorption speed is fast due to its large surface area. In addition, there are problems in that the use of an insoluble inorganic fine powder as a mixing-promotor, generally, results not only in the disadvantage of cost, but also in the formation of dust from the insoluble inorganic fine powder or in the deterioration of the granulation strength or the physical properties.
The present inventors found that there are problems in that even if water-absorbent resin powders are granulated using conventional granulating machines or methods, excellent absorption properties as expected cannot be maintained in final products, probably, due to destruction of granulation in conveyance steps of the water-absorbent resins or in processing steps to the final products (for example, paper diapers).
Furthermore, the inventors found that there might been seen physical property deteriorations, such as lowering of the absorption speed, increasing of water-soluble components as impurities, or lowering of the absorption capacity under a load, as a result of regeneration of fine powders due to the above-mentioned destruction of granulation, and further that, on the other hand, the inherent properties of the water-absorbent resin deteriorate when a granulation strength is increased by increasing the amount of an aqueous liquid, which is a binder, for the purpose of avoiding the destruction of granulation.
For example, fluidized-bed type mixers (EP 0318989) or high-speed stirring type mixers (U.S. Pat. No. 5,140,076), as conventionally used for granulation, provide inferior results in that the amount of an aqueous liquid as added to a water-absorbent resin powder is only several % up to at most 30%, and that it is very difficult to continuously and stably make granulation with the amount of the addition over 30%.
Furthermore, as to the conventional granulation methods, in the case where the amount of the addition of the aqueous liquid is larger than 30%, the mixing of the aqueous liquid and the water-absorbent resin powder is extremely non-uniform, and the physical-property deterioration or particle destruction occurs due to the non-uniform addition of the aqueous liquid. Thus, there is a limitation in the amount of the addition of the aqueous liquid for improving the granulation strength.
In addition, by the present inventors' study, it was found that: mixers with great kneading power, as conventionally used as means other than granulation, such as shearing mixers (EP 0417761) and Nauta type mixers, relatively facilitate the addition of the aqueous liquid, but provide inferior results in that a mixture resultant from the addition of the aqueous liquid does not form a granule, but merely forms a united large mass of a gel, and that the water-absorbent resin itself is deteriorated due to the shearing force of the mixers.
In addition, the present inventors further found that conventional processes, such as a process comprising granulation after crosslinking the surface neighborhood of a water-absorbent resin and a process comprising the simultaneous steps of the granulation and the surface-crosslinking of the water-absorbent resin, inevitably involve surface-crosslinking fracture due to the granulation, in other words, that water-absorbent resin compositions as obtained by the conventional granulation processes can bear only a low load of at most about 20 g/cm.sup.2 because of the fracture due to the granulation and display only a low absorption capacity of ten and several g/g under a high load of 50 g/cm.sup.2.
In addition, the present inventors further found that a water-absorbent resin primary particle alone, as obtained by removing the fine powder by classification, is not only economically disadvantageous because of the removal of the fine powder, but also slow in water absorption speed because of its small surface area, and further that a granule particle alone involves complicated process steps and is inferior because of factors such as gel fracture.